The present invention relates to a display panel, and particularly relates to a method and an apparatus for manufacturing a display panel in which an amorphous or poly-crystalline semiconductor film formed on an insulating substrate constituting the display panel is irradiated with a laser beam so as to be formed into a semiconductor film whose film quality is improved or whose crystal grains are enlarged, so that the properties of active devices such as thin film transistors both in a pixel region and in a peripheral circuit region can be optimized to display with high definition and high quality.
In a so-called active matrix display panel of a liquid-crystal display, an organic electro-luminescence display or the like, active devices such as thin film transistors formed out of a semiconductor film are disposed on one (active substrate or thin film transistor (TFT) substrate) of substrates constituting the panel. For example, in a liquid crystal panel in which liquid crystals are enclosed between two substrates preferably using glass, fused quartz or the like, a large number of pixels to be driven by thin film transistors formed out of an amorphous silicon film on the substrate are arrayed in a matrix, and the pixels are switched on/off by the thin film transistors so as to form a two-dimensional image. The thin film transistors for driving the pixels on the substrate are also referred to as “pixel transistors”. Then, a display signal is supplied to the pixel transistors selected by a drive circuit (hereinafter also referred to as “driver circuit”, whereas thin film transistors constituting the driver circuit are also referred to as “driver transistors”) placed in the periphery (peripheral circuit portion) of the substrate. The display signal is applied to a pixel electrode connected to an output electrode of each of the selected pixel transistors. Thus, the pixels corresponding to the selected pixel transistors are lit.
Currently, the peripheral circuit including the driver circuit is typically mounted in the periphery of the pixel region (display region) of the substrate in the form of an integrated circuit chip. If the driver circuit can be formed in the peripheral circuit portion of the substrate together with the pixel transistors, a so-called system-in panel can be realized so that exponential reduction in manufacturing cost and improvement of reliability can be expected. In the present circumstances, however, the silicon film (semiconductor film) forming the active layer of transistors is so poor in crystallinity that the performance of thin film transistors represented by mobility is low. Thus, it is difficult to manufacture a circuit required to have a high speed and a high function. In order to manufacture such a high-speed and high-function circuit, high-mobility thin film transistors are required. In order to realize the high-mobility thin film transistors, it is necessary to improve the crystallinity of the silicon thin film.
As a method for improving the crystallinity, excimer laser annealing has got a lot of attention in the background art. According to this method, an amorphous silicon film (whose mobility is not higher than 1 cm2/Vs) formed on an insulating substrate (hereinafter also referred to as “substrate” simply) of glass or the like is irradiated with excimer laser so that the amorphous silicon film is transformed into a poly-crystalline silicon film. Thus, the mobility is improved. However, in the poly-crystalline silicon film obtained by irradiation with excimer laser, the grain size is about several hundreds of nanometers and the mobility is about 100 cm2/Vs. Thus, the poly-crystalline silicon film is deficient in performance to be applied to a driver circuit or the like for driving a liquid crystal panel.
As a solution to this problem, there has been proposed an annealing technique using continuous-wave laser as disclosed in F. Takeuchi et al. “Performance of poly-Si TFTs fabricated by a Stable Scanning CW Laser Crystallization”, AM-LCD '01 (TFT4-3) or a technique in which a crystal nucleus generated by excimer laser is grown as a crystal with harmonics of a pulsed YAG laser as disclosed in JP-A-2002-270505.